The present invention relates to a discharge gas treatment apparatus and method for use with a crystal growing chamber utilizing the Czochralski method, such invention comprising a water spray apparatus capable of limiting the amount of Si compounds released into the atmosphere while avoiding safety hazards related to unwanted siphoning of water into the Czochralski chamber.
As industry drives demand for ever larger and higher quality crystals for use in microchip fabrication, crystal growers embrace an ever increasing body of technology which enables them to make crystals of increasing size and at an increasing rate. However, very real safety concerns still plague crystal growers as the techniques of today still require the control of potentially hazardous chemicals within a harsh high-temperature environment. Though hazardous waste streams produced during crystal growth are handled adequately by technology of today during its normal operation, the possibility of equipment failure mixed with the potentially extreme consequences of such an event could still be improved.
The majority of crystals today, particularly silicon crystals, are grown using the Czochralski method, known simply as the CZ method. In the CZ method, polycrystalline material is placed within a crucible along with any necessary dopants or other additives. The crucible, in turn, is placed within a closed chamber capable of being isolated from the atmosphere. Because of the extreme heat necessary for crystal production the primary components within a CZ chamber are typically graphite, with the exception of the crucible which is typical quartz.
The polycrystalline material is then heated and melted at around 1400xc2x0 C., after which a seed crystal having the desired orientation is placed just above the surface of the melted polycrystalline material, known as the melt. Because of surface tension, molten polycrystalline adheres to the lower surface of the crystal. As the molten polycrystalline along the lower surface of the crystal cools, it adopts the crystalline structure of the seed crystal. As the seed crystal is slowly raised within the Czochralski chamber, the crystal is grown as additional molten polycrystalline adheres to the crystal and solidifies with the proper crystalline orientation. The practice continues until a crystal of suitable size is achieved. Also, it must be noted that the CZ method usually involves constant rotation of the crystal and/or the crucible to counteract problems associated with non-uniform distribution of dopants and additives within the melt.
During the production of silicon crystals, various hazardous chemicals can be produced. Perhaps the most hazardous byproduct of high temperature crystal production is silicon oxide (SiO) which is believed to be produced by reaction of the polysilicon melt with the quartz crucible. The silicon oxide is violently reactive with oxygen to form SiO2. Though oxygen is normally evacuated from the CZ chamber during the crystal growing process, the mere existence of SiO within the CZ chamber or within a gas discharge stream is cause for concern since any accidental breach of the CZ chamber during the crystal growing process would result in the potentially explosive oxidation of SiO.
Other byproducts of the crystal growing process are SiO2, which is partially vaporized and may thereafter solidify above the melt, and SiC, which forms as a solid residue upon the graphite components of the CZ chamber, and may thereafter flake off into the area above and around the melt. Although SiO2 and SiC are relatively non-reactive, they are undesired components within the CZ chamber because of their ability to cause buildup within the chamber or, worse, to contaminate the melt and destroy the crystallinity of the growing crystal.
In order to minimize the adverse impact of the hazardous chemicals, and further to inhibit the production of the undesired byproducts discussed above, the CZ chamber is filled with an inert gas. Furthermore, crystal growth typically occurs under a slight vacuum.
In practice, the CZ chamber is repeatedly evacuated and supplied with inert gas as each crystal is produced. For instance, to insert a seed crystal within the CZ chamber, the chamber must necessarily be exposed to the atmosphere. After the seed crystal is put in place, the chamber is evacuated through use of a vacuum pump. Once the chamber has been evacuated, inert gas is supplied to the chamber, which is still under a slight vacuum. As the crystal growing process begins and continues, inert gas is continuously supplied to the chamber and continuously removed from the chamber through use of another vacuum pump. After the crystal has grown, the gas supply and discharge are closed, the chamber is opened to the atmosphere to allow access to the crystal, the melt is replenished if necessary, a new seed crystal is inserted, and the process is repeated.
The gas discharged from the chamber carries with it the various byproducts of the reaction, including SiO, SiO2, and SiC. As mentioned above, SiO is a potentially explosive component which is particularly dangerous when held in a hot, dry gas stream. The other components are also environmentally regulated and must be removed from the discharge stream before it is exhausted to the environment or otherwise reused.
Conventional discharge gas treatment methods often use a water bubbler to oxidize the SiO under controlled wet conditions, to solidify the SiO2 and SiC components of the gas stream, and to cool the discharge gas. Use of a water bubbler involves discharging the contaminated gas stream into a water filled tank. After the discharge gas enters the tank, the gas is rapidly cooled as the gas rises through the water. As the gas is cooled, a large percentage of the SiO2 and SiC precipitates out of the gas stream and falls to the bottom of the water bubbler, where it may be collected. The SiO within the gas stream reacts, in a controlled manner, with the oxygen present in the water to form SiO2, which precipitates to the bottom of the bubbler. The remaining gas stream is removed from the bubbler, still under slight vacuum, and either exhausted to the atmosphere or treated further.
Under normal operating conditions, the bubbler arrangement works quite well for removing contaminants from the gas stream. However, the CZ apparatus is operated at extreme temperatures and byproducts of the process, particularly SiO2, tend to solidify and readily form deposits within the discharge system which includes piping, vacuum pumps, and valves. Having SiO2 deposits throughout the gas discharge system creates a likelihood that a vacuum pump will fail or that a valve will be unable to close during crystal growth. Furthermore, the gas discharge system including the vacuum pump and the valves may fail for some other reason. Given that the crystal growing process is conducted under a vacuum, it is possible that, due to equipment failure, water from the water bubbler could be siphoned back through the gas discharge system, perhaps invading the vacuum pump or even the CZ chamber itself. The most likely cause of such siphoning would be failure of a discharge gas vacuum pump or failure of the vacuum seal between the gas discharge system and the CZ chamber during evacuation. A result of water backflow is at least destruction of the pump or the crystal being currently grown, and could perhaps result in a steam explosion caused by large quantities of water encountering the 1400xc2x0 C. chamber atmosphere.
Previous attempts to overcome the problem of water backflow have involved various methods of mechanically and physically isolating the water bubbler from the CZ chamber and vacuum pumps. Two approaches to isolating the bubbler from the chamber are found in U.S. Pat. No. 5,900,058, which discloses the use of a buffer tank between the CZ chamber and the bubbler. The buffer tank is a gas filled tank located in-line with the gas discharge system, between the bubbler and the CZ chamber and any vacuum pumps. In theory, any water siphoned back into the discharge system would be captured by the buffer tank before it was able to make its way into pumps or the CZ chamber. The same patent also discloses use of a vacuum breaker device which would be able to sense loss of vacuum within the gas discharge zone of the Czochralski apparatus and which would quickly bring the CZ chamber to atmospheric pressure thereby stopping any siphoning from taking place.
Conventional safety devices have lowered the possibility of catastrophic equipment failure during crystal growing using a CZ chamber. However, the safety measures related to use of the water bubbler are far from failsafe. Rapid buildup of SiO2 and SiC deposits within the gas discharge zone means that equipment failure can occur, including equipment failure of the safety equipment mentioned in the prior safety techniques.
It would, therefore, be desirable to provide an apparatus and method of preventing water backflow into a CZ apparatus upon vacuum pressure disruption, wherein such apparatus is not subject to the same disabling deposits and corrosive effects of the CZ environment which typically cause the disruption. In other words, an improvement in safety during discharge gas treatment with water is desirable which does not involve exposing the safety equipment to the same disabling environment which likely caused an equipment failure in the first place.
The present invention provides an apparatus and method of treating discharge gas from a CZ chamber which dramatically reduces the risks of water backflow associated with traditional water bubblers. The reduction of risk of water backflow is achieved by replacing the traditional water bubbler with a water spray subsystem within the gas discharge system of the CZ apparatus. The water spray subsystem provides adequate interaction between the discharge gas and the cooling, reacting water, so that the SiO in the gas stream reacts to SiO2, and so that the SiO2 and SiC are cooled and precipitated out of the gas stream. But, the water spray subsystem works without the necessity of a large accumulated volume of water that is maintained in communication with the CZ chamber, thus avoiding the possibility of water backflow and of all the catastrophic consequences related thereto.
The water spray subsystem is designed for use in the gas discharge zone of a Czochralski crystal growing apparatus. The subsystem composes a structure containing or defining a water spray body, which is capable of providing a water spray into the discharge gas stream from the CZ chamber. The water is supplied into the discharge gas in the form of a spray or a mist in order to provide for intimate contact between the droplets of sprayed water and the discharge gas stream. Contacting the discharge gas stream with the water spray cools the gas stream, causes the reaction of SiO to SiO2, and causes the precipitation of cooled SiO2 and SiC out of the discharge gas stream.
In an embodiment, the subsystem is composed of a closed vessel having a discharge gas inlet for receiving discharge gas containing various waste components, from the CZ crystal growing chamber, and a discharge gas outlet which allows the wetted, cooled, and reacted discharge gas to leave the vessel and continue to another treatment process or to exhaust into the atmosphere. The spray body is preferably located within the vessel and provides a water spray to the inside of the vessel. There is at least one drain to allow for the escape of water from the vessel.
The vessel preferably has an upper region and a lower region with the upper region having a central area and a periphery, such as a cylindrical tank having a vertical axis of orientation or a conical tank having a vertical axis of orientation. The gas inlets and outlets are disposed within the upper region of the vessel, so that condensing water from the spray body naturally falls away from and is not sucked into either the inlet or outlet. The water spray body is disposed about the periphery of the upper region of the vessel so that water sprayed into the vessel is projected inward and downward into the vessel. The spray body most preferably is composed of a series of spray heads which provide a very dispersed water spray having a wide spray pattern within the vessel. The condensed water, along with the particulates condensed out of the discharge gas, are removed from the vessel through a water outlet located in the lower region of the vessel.
In operation, discharge gas is projected downwards into the main volume of the vessel where it intimately contacts water which is likewise projected downward and inward into the main volume of the vessel. Upon contact of the discharge gas with the water, SiO reacts with oxygen present in the water to form SiO2, and the SiO2 and SiC within the gas stream are cooled and solidified, after which they fall to the bottom of the vessel. The cooled and reacted gas stream leaves the vessel through the gas outlet. The condensed water and particulates within the vessel settle to the bottom of the vessel, where they are removed through a water outlet.
By utilizing the water spray of the invention, adequate cooling and reaction of waste products within the discharge gas stream from the CZ chamber are provided.
Furthermore, use of the water spray and consequent lack of any large accumulated volume of water avoids the possibility of water being siphoned back into the CZ chamber in the event of an equipment failure. By eliminating the volume of water normally present in a bubbler, the present invention provides a dramatic improvement in safety by eliminating the possibility of water backflow into the discharge gas treatment system which has previously presented a hazard of mechanical destruction or steam explosion.